Nuclear radiation gauges have been widely used for measuring the density and moisture of soil and asphaltic materials, or other construction material. As used herein, construction material is any materials used in building roads or foundational structures including, but not limited to soils, asphalts, asphalt-like materials, concrete, composite materials, or the like. Such gauges typically include a source of gamma radiation which directs gamma radiation into the test material, and a radiation detector located adjacent to the surface of the test material for detecting radiation scattered back to the surface. From this detector reading, a determination of the moisture and density of the material can be made.
These gauges are generally designed to operate either in a “backscatter” mode or in both a backscatter mode and direct transmission mode. In gauges capable of direct transmission mode, the radiation source is vertically moveable from a backscatter position, where it resides within the gauge housing, to a series of direct transmission positions, where it is inserted into small holes or bores in the test specimen.
Many of the gauges commonly in use for measuring density of soil, asphalt and other materials are most effective in measuring densities of materials over depths of approximately 3 to 12 inches. However, with the increase in cost of paving materials, the practice in maintaining and resurfacing paved roadbeds has become one of applying relatively thin layers or overlays having a thickness of one to three inches. With layers of such a thickness range, many density gauges are ineffective for measuring the density of the overlay because the density reading obtained from such gauges reflects not only the density of the thin layer, but also the density of the underlying base material.
Nuclear gauges capable of measuring the density of thin layers of materials have been developed by Troxler Electronic Laboratories, Inc. of Research Triangle Park, N.C. For example, thin layer density gauges are disclosed in U.S. Pat. Nos. 4,525,854, 4,701,868, 4,641,030, 6,310,936 and 6,442,232, all of which are incorporated herein by reference in their entirety. Some of the gauges disclosed in the above-referenced patents are referred to as “backscatter” gauges because the radiation source does not move outside the gauge housing, which is necessary for measurement in the direct transmission mode. In some of the gauges disclosed in the above-referenced patents, the gauge can have radiation sources that can also be extended outside of the gauge housing and into the material to be measured in a direction transmission mode. Typically, the source rods can extend up to about 12 inches.
As disclosed in the above patents, the preferred method of measuring the density of thin layers of materials, such as asphalt, is nondestructive and uses the backscatter mode. One method requires two independent density measurement systems. The geometry of these two measurement systems must be configured with respect to one another and with respect to the medium being measured in such a manner that they measure two different volumes of material. The two different volumes are not mutually exclusive insofar as they partially overlap one another. Measurement accuracy depends upon a larger portion of the volume measured by one of the measurement systems being distributed at a lower depth beneath the gauge than the volume measured by the other measurement system. This is accomplished by placing one radiation detection system in closer spatial proximity to the radiation source than the other detection system. Another volume specific measurement is typically used in soils and requires drilling a small hole in the material under test. This method is referred to as the direct transmission mode.
To provide proper position of the movable radiation source in some nuclear gauges, index positions were cut mechanically into the source rod. This source rod is problematic to precisely set and adjust and adds to the overall product cost. In other nuclear gauges, the means for extending and retracting included an index rod operatively positioned adjacent to the source rod. The index rod included a plurality of notches. Each notch corresponds to a predetermined source rod position. For example, one notch corresponds to the “safe” position wherein the radiation source was raised and shielded from the test material. Other notches can correspond to positions that place the source up to about 12 inches in the material. These indexing rods have been also been problematic to set and also add to the overall product cost.
There remains a need in the art for a nuclear gauge capable of operating in backscatter mode and/or direct transmission mode, and which is suitable for measuring the density and moisture of construction material.